Dicoumarol is a naturally occurring anticoagulant that functions as a functional vitamin K depleter. It is also used in biochemical experiments as an inhibitor of reductases. Dicoumarol is a natural chemical substance of combined plant and fungal origin. It is a derivative of coumarin, a bitter tasting but sweet-smelling substance made by plants that does not itself affect coagulation, but which is (classically) transformed in mouldy feeds or silages by a number of species of fungi, into active dicoumarol. Dicoumarol does affect coagulation, and was discovered in mouldy wet sweet-clover hay, as the cause of a naturally occurring bleeding disease in cattle. Like all 4-hydroxycoumarin drugs it is a competitive inhibitor of vitamin K epoxide reductase, an enzyme that recycles vitamin K, thus causing depletion of active vitamin K in blood. This prevents the formation of the active form of prothrombin and several other coagulant enzymes.